The present disclosure relates generally to an apparatus and method for filtering current sensor output signals, and particularly to an apparatus and method for filtering the output signals of a Rogowski coil type current sensor.
Electronic circuit breakers with electronic trip units often employ a Rogowski coil current sensor for producing a signal representative of the primary current passing through the circuit breaker to a protected circuit downstream of the circuit breaker. The current sensor output signal is passed to a processing circuit within the trip unit where an integration routine is performed for rms (root-mean-square) analysis of the primary current. Short-time and long-time trip sequences are initiated by the trip unit in response to the rms analysis indicating an over current and/or over heating condition in the protected circuit. An instantaneous trip sequence may also be initiated at the trip unit in response to the current sensor output signal indicating an instantaneous over current condition in the protected circuit in excess of an instantaneous trip threshold. However, since a Rogowski coil produces an output voltage that is proportional to the d(i(t))/dt of the primary current, a primary current having a harmonic content may result in an output signal from the Rogowski coil that is not accurately representative of the actual instantaneous primary current. In an effort to resolve such inaccuracies in the instantaneous trip regime of electronic circuit breakers, supplemental magnetic trip systems may be employed, which adds complexity and cost to the design of such circuit breakers. Accordingly, there remains a need in the art for an electronic circuit breaker that provides for a greater degree of accuracy in the instantaneous trip regime where Rogowski coil type current sensors are employed in situations where the primary current content may include higher order harmonics.